This application relates to and incorporates herein by reference Japanese Patent Application No. 2000-119530 filed. Apr. 20, 2000.
The present invention relates to a filter device having adjustable filtering characteristics, that is, an adjustable frequency response, and a method of adjusting the central frequency of the pass-band of the filter device.
In mobile telecommunications using high frequency waves, filter devices are used to pass only signals of predetermined frequencies and cut off other signals of other frequencies. The filter device generally employs a dielectric-type filter or a cavity resonator-type filter. Those filter devices are constructed to maintain the filtering characteristics (frequency response) thereof, even when the operating temperature near the room temperature changes. The filtering characteristics are usually adjusted by changing the resonance frequency of each resonator in the filter device or changing the coupling among the adjacent resonators by way of screws or the like. It is however impossible to adjust the filtering characteristics once the filter device has been installed in a closed-type mechanical apparatus, for instance, in a mobile telecommunication base station.
It is an object of the present invention to enable adjustment of filtering characteristics, that is, a frequency response, of a filter device even after installation in a closed mechanical apparatus.
According to the present invention, a filter device includes filters connected in series with each other. Operating temperatures independently of each other. Each filter varies its filtering characteristics (frequency response), particularly its central frequency of pass-band width, in correspondence with the operating temperature, while maintaining the same pass-band width. As the filters operated at the different operating temperature provide different filtering characteristics, the combined or resulting filtering characteristics of the filtering device can be adjusting as desired even after the filtering device is installed at a mobile telecommunication base station.
When a filter device includes only one filter, the filtering characteristics, particularly the central frequency of its pass-band width, are adjusted by varying the operating temperature of the filter from outside of the filter device after installation at a telecommunication base station.